The invention relates to a broadband distributed amplifier comprised of a plurality of field effect transistors whose gates are connected to inductors in such a manner as to form a gate transmission line, whose drains are connected to inductors in such a manner as to form a drain transmission line, and whose sources are connected to ground. The distribution of the amplifier stages along the gate and drain transmission lines is effected such that these lines are periodically loaded by their own impedances and by the transistor gate and drain capacitances thus forming artificial lines, and such that a micro-wave frequency input signal applied to the input of the gate transmission line effects production of an amplified microwave frequency output signal at the output of the drain transmission line. The gate and drain transmission line output and input, respectively, are terminated by loads, and at least one of these loads includes a biasing circuit.
The invention is used for the production of integrated amplifiers on semiconductor substrates of group III-V, which operate in the microwave-frequency range, over a broad band, for example between 2 and 30 GHz and higher.
Such a distributed amplifier is disclosed in the publication IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-32, No. 1, January 1984, in the article "2-20 GHz GaAs Traveling-Wave Amplifier" by YALCIN AYASLI et al., pages 71 to 78.
The above-mentioned document describes that this amplifier is formed by a plurality of cascaded stages. Each stage includes a field-effect transistor whose input and output capacitances are combined with inductors for forming an input line and an output line, respectively. These inductors are of the micro-strip type and consequently are periodically charged by gate and drain impedances of the field-effect transistors.
Such a circuit has the property of having a very broad band, extending to the microwave-frequency range, and of being integrable only on, for example, gallium-arsenide. But as described in the AYASLI publication the amplifier has certain disadvantages. More specifically, its maximum operating frequency is limited because of the biasing circuits which degrade performance. According to the publication, this problem is solved to the best possible extent by sophisticated gate and drain biasing circuits provided at the ends of the artificial transmission lines and constituting line sections and capacitances forming LC circuits.
However, when a person skilled in the art wants to implement the circuit described in the publication, it appears that, in spite of the improvement in the biasing circuits, multiple reflections are produced at the ends of the gate and drain lines which still limit the passband of this amplifier. Because of the coupling between the gate line and the drain line due to gate-drain capacitances of the transistors, the reflections disturb the propagation along the gate and drain lines. It further appears that the variations in the characteristic impedances of the artificial transmission lines versus frequency cannot be accurately compensated for by effecting a simple improvement in the circuits, the biasing circuits included.